Rust-proof properties have conventionally been imparted to cold-rolled steel sheets, galvanized steel sheets, zinc-based alloy-plated steel sheets and aluminum-plated steel sheets used for automobiles, electrical appliances, building materials and the like, usually by coating their surfaces with chromate layers. Chromating treatment includes electrolytic chromating and application chromating. Electrolytic chromating is accomplished, for example, by using a bath composed mainly of chromic acid and also containing other anions such as sulfate, phosphate, borate and halogens, for treatment of the metal sheet by cathodic electrolytic treatment. Application chromating is designed in consideration of the problem of elution of chromium from chromated metal sheets, and it thus involves preparation of a treatment solution by adding an inorganic colloid or inorganic anion to a solution with a portion of the hexavalent chromium portion reduced to trivalent chromium beforehand or to a solution with a specified ratio of hexavalent chromium to trivalent chromium, and immersing the metal sheet therein or spraying the metal sheet with the treatment solution.
Of such chromate layers, those chromate layers formed by electrolysis cannot be said to have sufficient corrosion resistance despite the low elution of hexavalent chromium, and there is particular loss of corrosion resistance in cases where considerable layer damage occurs during working, etc. On the other hand, while metal sheets coated with application chromated layers have high corrosion resistance and especially high excellent corrosion resistance of worked sections, elution of hexavalent chromium from the chromate layers has been a problem. Elution of hexavalent chromium can be considerably reduced by coating with organic polymers, but this is still inadequate. Although an improvement in reducing elution of hexavalent chromium can generally be achieved by a method known as resin chromating treatment, such as disclosed in Japanese Unexamined Patent Publication No. 5-230666, it is still impossible to avoid trace elution.
Thus, in order to completely inhibit elution of hexavalent chromium it is necessary to develop a corrosion-resistant layer that uses absolutely no hexavalent chromium but has functions equivalent to conventional chromate layers containing hexavalent chromium.
One previous anti-corrosion technique for incorporating absolutely no hexavalent chromium is a method under development which uses an organic-based corrosion inhibitor. As such organic-based corrosion inhibitors there are known carboxylates such as benzoates, azelates, etc. and compounds containing --S--, --N-- which readily interact with metal ions, as well as complexes thereof.
As techniques for including organic-based corrosion inhibitors in layers there have been proposed, for example, the hydrooxime complex of zinc disclosed in Japanese Unexamined Patent Publication No. 62-23989, the metal chelate compounds of Mg, Ca, Ba, Zn, Al, Ti, Zr, Sn, Ni, etc. disclosed in Japanese Unexamined Patent Publication No. 3-183790 and Japanese Unexamined Patent Publication No. 2-222556, the alkali earth metal salts, transition metal salts and transition metal complexes of organic compounds disclosed in Japanese Unexamined Patent Publication No. 6-321851, and the titanium and zirconium complexes of carboxylic acids disclosed in Japanese Unexamined Patent Publication No. 8-48916. These corrosion inhibitors, however, have weak anti-corrosion effects due to the metal elements forming the complexes and thus have failed to provide the same function as hexavalent chromium. In particular, almost no corrosion resistance can be expected at damaged sections or at the locations of layer defects produced during working.
Japanese Unexamined Patent Publication No. 7-188951 discloses a rare earth metal-organic chelate compound for the purpose of inhibiting corrosion of metals that contact solutions, such as radiators or pipes. This corrosion inhibitor was designed as a water-soluble compound, to allow continuous provision of the corrosion inhibitor to corrosion sites by circulation of the solution. Consequently, although the strong anti-corrosion effect of the rare earth metal element is utilized, with layers on metal sheets wherein the absolute amount of corrosion inhibitor onto the corrosion sites is limited by the coating coverage, elution occurs out of the layer in humid atmospheres so that long-term corrosion resistance comparable to chromate layers cannot be achieved.